It is known to use a continuous press for the lamination of a metal foil constituting one of the outer layers or the outer layer of the structure and at least one layer of glass fiber containing a thermally hardenable synthetic resin by continuously passing the layers through the press and subjecting the layers in the press to an appropriate pressure heating to activate the thermally hardenable binder and, if desired, cooling.
Two metal foil webs can flank a multiplicity of layers of the glass fiber impregnated with the thermally hardenable synthetic resin where the resulting laminated structure is to have conductive metal surfaces flanking an electrically insulating core.
As indicated, it is known to continuously produce the hard structure for use in the fabrication of circuit boards in a continuously operating press which can have, at the mouth thereof, a pair of drums over which respective press belts are guided, the press belts defining the mouth and entraining the layers through the mouth into a pressing gap between pressing plates or tables, for example, provided with heating means and, if desired, with cooling means downstream from the heating means. The cooling can, of course, be effected downstream of the continuous throughput press.
Within the press and in the pressing region, i.e. the gap, the layers are subjected to compaction and heating to activate the thermosetting binder, thereby producing a rigid composite strip which emerges from the press for possible cooling and further circuit board fabrication techniques.
A system of this type is described in the Siempelkamp Bulletin of May 1983, pages 4 and 9 and Section 3.2.
Also, as background to the present invention, attention may be directed to German Patents Documents No. 31 19 529 and 34 32 548 which describes continuous presses for the production of laminated products in a continuous manner as described above.
When reference is made herein to glass strips, it will be understood that I intend to so identify woven and nonwoven glass fiber webs, generally impregnated with thermally activatable synthetic resins. The thermally hardenable or thermosetting resins can be epoxy resins, phenolics and any of the other thermally hardenable resins which have been used in the fabrication of printed circuit boards heretofore.
The metal foils which are used are usually copper foil webs.
With respect to the resins which are employed, reference can be made to the Handbuch der Leitertplattentechnik, 1982, pages 25-52 in which the starting webs or layers are so-called prepregs and in which glass fiber webs are impregnated with epoxy resins in the so-called B-state. It is not necessary, using such prepregs to subject the glass fiber webs or metal foil webs to any special thermal treatment as long as the compression and temperature conditions in the press are capable of forming the rigid and consolidated product.
It has been found, however, that problems have been encountered in the past in the processing of extremely thin metal foils through such presses and in the prior art processes.
In particular, mention can be made of the fact that when the laminated structure is to be produced at a high rate and though the metal foil may be used at a high velocity to the press, frequently the foil will wrinkle, double-up, fold, tear or distort. While these drawbacks can be avoided by the use of thick foils, frequently thick foils are not desirable.
The same drawback applies to the system of U.S. Pat. No. 4,670,080 in which prepregs which do not require prior thermal treatment upon entry into the press are not used, but rather the press may be equipped to carry out a pressureless preheating of the glass fiber strip to the point that the epoxy resin system will no longer be in the B-state at the time of consolidation.